1. Field of the Invention
The present invention relates to portable security alarm systems and, in particular, to alarm systems with coded or secure disarm methods designed for use in isolated locations with minimum power requirements.
2. Prior Art
There is a wide variety of portable security alarm systems known to the prior art. These systems may employ audio and visual alarm functions. There are two principal disadvantages with existing systems. First, the alarms are of the low volume type because they are adapted for use in areas where they will be within hearing distance of friendly personnel. These systems are therefore unsuitable for use in isolated or unmonitored areas devoid of friendly personnel. Second, the arm and disarm methods employed are usually a simple mechanical switch to arm and disarm the system by turning off the power to the system. This arrangement allows any one with access to the system to disable the alarm functions.
What is most desired in a security system for unmonitored areas where friendly people are not close to the system is both a very loud alarm that can be heard some distance away and a secure disarming method that allows only a limited number of authorized persons to disable the alarm function. In addition, the system should be operable via a self-contained power supply for a substantial period of time. The present invention is specifically designed to accommodate the conflicting objectives of a very loud alarm with minimum power requirements. The alarm system should include design features that recognize the possibility of electrical storms and resulting circuit interference such as false alarms. Finally, the system should have the capability of rearming itself to the monitoring mode after one or more alarm cycles as well as providing indication that an alarm condition has occurred.
Representative alarm systems of the prior art include U.S. Pat. Nos. 4,151,520 (Full) and 4,335,376 (Marquardt). In the Full device, once the alarm is activated, it will remain on until the arming switch is turned off or until the batteries run down. The Marquardt device can also run down unless the triggering mechanism, in this case a door switch, is reversed by closing the door. The Marquardt device can then be reactivated by a subsequent intrusion. Neither device includes either circuitry to minimize power consumption or a secure disarming method.
In Helft, et al., U.S. Pat. No. 4,422,068, an indication of a prior alarm actuation is provided by an indicating light. This is a useful feature but it is not accompanied by circuitry to return the system to standby or monitoring after the alarm function has been terminated. The Helft device also lacks a secure disarming method.
None of the devices in the prior art deal with the need to minimize the power requirements of the system nor do they include specific design features to minimize spurious electrical transients resulting from electrical storms and the like.